As prior art solid image sensors, CCD or MOS image sensors have been used. While these image sensors are useful for some applications, it is difficult to design them suitable for photo-copying machine in the form of a linear strap which can span the length of an original paper. A number of sensor segments arranged along the linear strap can indivusually detect illumination.
In recent years, the development of contact image sensors has been broadly the subject of researchers. Image sensors of this type are provided with fine structure on a broad substrate in which eight sensors are arranged across each 1mm width. One of the difficulties in producing such a fine structure is appropriate designing of etching process. Wet-etching having a short process time is not suitable since micro-scale treatment must be done on a wide substrate. For this reason, dry-etching has been employed instead. In the later case, it is very difficult to etch a thin semiconductor film throughout the wide area of a substrate.